I am not sure that you can update after you have submitted the ERAS. You could try the EDIT function but once it's gone, it's gone. Take heart though, your Deans Letter is definitely going to mention your selection to AOA so it will be in your application at least in one spot.
I had to do this a couple of weeks ago. Here's what you do:
1. Log in.
2. Under the "Account' tab select "profile".
3. Scroll to the bottom and select "Member of AOA".
4. Hit "Save".
5. It will create a new profile for your programs to download.
6. Congrats.
